Donor-acceptor-substituted tetrakis(phenylethynyl)benzenes as emissive molecules during pulse radiolysis in benzene

Emission from charge recombination between radical cations and anions of various tetrakis(phenylethynyl)benzenes (TPEBs) was measured during pulse radiolysis in benzene (Bz). The formation of TPEB in the singlet excited state ((TPEB*)-T-1) can be attributed to the charge recombination between TPEB center dot+ and TPEB center dot-, which are initially generated from the radiolytic reaction in Bz. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (-Delta H degrees) for the charge recombination of TPEB center dot+ and TPEB center dot- and excitation energy of (TPEB*)-T-1. It was found that the charge recombination between TPEB center dot+ and TPEB center dot- occurred to give (TPEB*)-T-1 as the emissive species, but not the excimers because of the large repulsion between substituents caused by the rotation around C-C single bonds of TPEBs. Since donor-acceptor-substituted TPEBs possess three types of charge-transfer pathways (linear-conjugated, cross-conjugated, and "bent" conjugated pathways between the donor and acceptor substituents through the ethynyl linkage), the emission spectra of (1)TPEBs(*) with intramolecular charge transfer (ICT) character depend on the substitution pattern and the various kinds of donor and acceptor groups during pulse radiolysis in Bz.